Relativity Definition and 998 Threads

The theory of relativity usually encompasses two interrelated theories by Albert Einstein: special relativity and general relativity, proposed and published in 1905 and 1915, respectively. Special relativity applies to all physical phenomena in the absence of gravity. General relativity explains the law of gravitation and its relation to other forces of nature. It applies to the cosmological and astrophysical realm, including astronomy.The theory transformed theoretical physics and astronomy during the 20th century, superseding a 200-year-old theory of mechanics created primarily by Isaac Newton. It introduced concepts including spacetime as a unified entity of space and time, relativity of simultaneity, kinematic and gravitational time dilation, and length contraction. In the field of physics, relativity improved the science of elementary particles and their fundamental interactions, along with ushering in the nuclear age. With relativity, cosmology and astrophysics predicted extraordinary astronomical phenomena such as neutron stars, black holes, and gravitational waves.

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  1. wolram

    B Is Space Truly Expanding or Just a Pedagogical Tool?

    arXiv:1605.08634 (cross-list from physics.pop-ph) [pdf] On The Relativity of Redshifts: Does Space Really "Expand"? Geraint F. Lewis Comments: 6 pages, 5 figures, appeared in Australian Physics Journal-ref: Australian Physics (2016), 53(3), 95-100 Subjects: Popular Physics (physics.pop-ph)...
  2. DaTario

    I Quantum Mechanics & Relativity: Vacuum Connection?

    Hi All, Once in the past I have heard that the vacuum in quantum mechanics, having its energy given by ## \sum_n \frac{1}{2}\hbar \omega_n ## was also obtained by methods of Einstein's relativity, through the claim that the vacuum should be a field which is invariant under a boost...
  3. P

    I Special relativity of two clocks

    Why is it that for two clocks that are synchronised in one frame, S, but not in another, S', is there an offset in the time by a factor of ##\frac{Lv}{c}##, as measured in S'. Where L is the proper length of the body, as measured in S. I'm confused as to why there is not a factor of ##\gamma##...
  4. i_hate_math

    Calc Time for Electron to Travel 100m in Lab Frame

    Homework Statement This is an extract of a very long question. The electron is moving relative to the lab at 0.98c. In the reference frame of the electrons, calculate the time required for an electron to travel the length of a beam pipe which has a proper length of 100 m measured in the...
  5. Gerald Kaiser

    Are temperature and pressure related by relativity?

    Since the temperature of a gas is related to its average molecular energy and the pressure to the average molecular momentum, it would seem that a Lorentz transformation would somehow relate the two. Does anyone know of related work?
  6. zdroide

    I Is there a real need for a theory of everything (ToE)?

    I been studying quantum field theory and standard model lately. I not see how a unified theory could fit between quantum field and special or general relativity. One being for big objects and the other for microscopic one. In fact, standard model not seem to be all proven. Many particles are not...
  7. Kara386

    Special relativity muon example

    Homework Statement Here's a standard example of special relativity in action: The mean lifetime of the muon as measured in a laboratory is about 2µs (rounded to 1 s.f.). Thus, the typical distance traveled by a muon should be about ##3\times 10^8ms^{-1}\times 2\times 10^6s = 600m##. The...
  8. Chris Frisella

    EM Phenomenon: Magnetism from a Cathode Ray?

    I have recently studied about relativity being the reason for magnetism. The example given was about a magnetic field generated from current down a long wire. When two such wires with current in the same direction were parallel to each other there was an attractive force between them because...
  9. V

    (Special relativity) Two masses connected by spring

    Homework Statement Hi all--humanist here. Now that the semester is over, I am taking the opportunity to (attempt to) self-study introductory SR. This is problem 12 in chapter 1 of Special Relativity by AP French. (1-12) A body of mass m1+Δm is connected to a body of mass m2-Δm by a spring of...
  10. philipp

    I Time dilation in special vs. general relativity

    Hi all, I was trying to understand the time dilation in special and general relativity and after much time of "overthinking" I am pretty much stuck now. My problem is, that what seems to me to be the same premises apparently imply opposite things. In special relativity, for two inertial...
  11. D

    A Interpretation of the EM tensor as a rotation matrix

    In special relativity, the electromagnetic field is represented by the tensor $$F^{\mu\nu} = \begin{pmatrix}0 & -E_{x} & -E_{y} & -E_{z}\\ E_{x} & 0 & -B_{z} & B_{y}\\ E_{y} & B_{z} & 0 & -B_{x}\\ E_{z} & -B_{y} & B_{x} & 0 \end{pmatrix}$$ which is an anti-symmetric matrix. Recalling the...
  12. i_hate_math

    When Will the Proton Meet the Rear of the Ship?

    Homework Statement The figure shows a ship (attached to reference frame S') passing us (standing in reference frame S) with velocity http://edugen.wileyplus.com/edugen/shared/assignment/test/session.quest2564447entrance1_N1002E.mml?size=14&ver=1463885870814 =...
  13. F

    I Does Relativity and Quantum Mechanics NEED to be reconcile?

    One of the greatest quest in physics is to reconcile Relativity with QM. But is this reconciliation really necessary? They both work quite well in their respective fields so why not just leave it at that? The only issue I can see is the problem of the singularity but can't that be solved by...
  14. i_hate_math

    Special Relativity of Train Problem

    Homework Statement A relativistic train of proper length 237 m approaches a tunnel of the same proper length, at a relative speed of 0.951c. A paint bomb in the engine room is set to explode (and cover everyone with blue paint) when the front of the train passes the far end of the tunnel (event...
  15. X

    Mathematica Best Mathematica Tensor/General Relativity Package?

    Hello can anyone recommend me a good mathematics package for solving the Einstein Field equations. You know one that can easily compute covariant derivatives and calculate the Reinman curvature tensor and can also minimize the Einstien Hilbert Action?
  16. J

    I Velocity of Timelike Curve in Special Relativity

    In special relativity we can view spacetime as ##\mathbb{R}^4## with its standard smooth structure, and a metric ##\eta_{ab} = \sum\limits_{\mu, \nu = 0}^3 \eta_{\mu, \nu} (dx^\mu)_a (dx^\nu)_b## where ##\nu_{\mu \nu} = \mathrm{diag}(-1, 1, 1, )##. Given a curve ##\gamma: I \rightarrow...
  17. Supantho Raxit

    Field of Moving Masses: EM Purcell

    A moving charge that starts or stops generates a spherical wave-front of EM wave, can the same analogy be drawn for a moving mass? Reference: Electricity & Magnetism by EM Purcell
  18. S

    Relativity - velocity of object travelling perpendicular to the observer

    Homework Statement Two light beams collide head on. Calculate their relative velocity. (c) A particle moves north at speed 0.85c relative to an observer standing on the Earth. What is the velocity of this particle as observed by a fast ship traveling east on the Earth at speed 0.9c? Give the...
  19. Kara386

    Special relativity question with lots of frames

    Homework Statement At exactly 00:00:00 hours, a group of convicts escape from a planet in a space-ship that travels at speed ##v=\frac{4}{5}c##. After 11 min, a patrol spaceship goes after them with ##v_P=\frac{24}{25}c##. Ignore all acceleration periods. (i) The convicts immediately notice the...
  20. S

    Calculating Velocity and Time Dilation in Special Relativity

    Homework Statement A spaceship travels from Earth to the vicinity of the star that is measured by astronomers on Earth to be six light-years away. The spaceship and its occupants have a total rest mass of 32 000 kg. Assume that the spaceship travels at constant velocity. The time taken as...
  21. F

    I Is motion through space or spacetime?

    I was chatting to someone recently about the motion of objects and whether they propagate through spacetime or not. They were/are convinced that motion through spacetime is simply not possible arguing something along the lines of the following: "Objects move through space. If you depict an...
  22. Stephanus

    B How does relativity impact our everyday lives beyond GPS and nuclear energy?

    Dear PF Forum, I'd like to have some opinions here. A friend of mine says, "From Newton , we got car, airplane, rocket, etc. From Einstein, we got sci fi movies". Okayy, forget him :smile: Now, what I want to know is, what relativity has direct impact in our everyday life? There are two things...
  23. M

    Derive p^2/2m from relativistic equations

    Given the relativistic equation for energy E2 = (pc)2 + (mc2)2 I want to find the non-relativistic approximation for kinetic energy in non-relativistic terms, Knr = p2/2m I start off with subtracting the rest energy E0=mc2 from the above equation. So K = E - E0 and assume that c is very...
  24. Approximation

    I Implications of Relative Simultaneity: Carl & Shirley's Observations

    Let's say you have a planet rigged to explode only if two bowling balls hit respective sensors at the same time. An unfortunate observer, call him Carl, releases two bowling balls at the same time from his reference frame. Meanwhile, Shirley passes by at nearly the speed of light. From Shirley's...
  25. F

    I Motivation for the usage of 4-vectors in special relativity

    I recently had someone ask me why we use 4-vectors in special relativity and what is the motivation for introducing them in the first place. This is the response I gave: From Einstein's postulates( i.e. 1. the principle of relativity - the laws of physics are identical (invariant) in all...
  26. P

    Centre of mass velocity special relativity

    Why is it that the ZMF velocity is given by the total momentum of the system divided by the total energy? Many thanks :)
  27. F

    I How can photons interact if they travel at the speed of ligh

    I was listening to a podcast about the solar neutrino problem, and they discussed how we have deduced that neutrinos are not massless due to the fact that they interact with other particles (even if this interaction occurs rarely). I paraphrase: "a particle traveling at the speed of light is...
  28. Zahidur

    Does light always travel at light speed?

    I've been told contradicting ideas about this. I've been told that light doesn't travel at a constant speed everywhere (i.e. light slowing down in speed after entering a more dense medium). However, I've also read that light speed is constant everywhere (i.e. if you could travel close to the...
  29. R

    Help Solve Physics Problem: Lamp Moving at v=c/4

    Hello, I need help with this problem. In 2D we have a lamp that emits light in 360 degrees around itself while stationary. Than it starts moving at velocity of c/4 what is the degree of the cone in which the light now emits to ? I hope you understand, like the angle of the field where the light...
  30. C

    B How can we tell that "fictitious forces" are not real?

    Earlier today I was reading through this entire thread: https://www.physicsforums.com/threads/implications-of-the-statement-acceleration-is-not-relative.670653/ And I remained confused about one thing in particular. The original poster made a statement (bolded below) on page 1 that seemed...
  31. little neutrino

    Relativity and Lorentz Transformations

    Homework Statement Spaceship A of length 30m travels at 0.6c past spaceship B. Clocks in frame S' of spaceship A and S of spaceship B are synchronised within their respective frames of reference and are set to zero, so that t' = t = 0 at the instant the front of spaceship A passes the rear of...
  32. L

    I Exploring General Relativity: A Beginner's Guide

    What is general relativity all about. I don't know anything about it. I am an undergrad.
  33. little neutrino

    The Classic Pole in Barn Relativity Question

    Homework Statement A pole-vaulter holds a 5.0 m pole. A barn has doors at both ends, 3.0 m apart. The pole-vaulter on the outside of the barn begins running toward one of the open doors, holding the pole level in the direction he is running. When passing through the barn, the pole just fits...
  34. little neutrino

    Relativistic Time Dilation and Length Contraction in Overtaking Rockets

    Homework Statement Two rockets are each 1000m long in their rest frames. Rocket A, traveling at 0.800c relative to the earth, is overtaking Rocket B, which is moving in the same direction at 0.600c. (i) According to the crew on B, how long does A take to completely pass? I.e. how long is it...
  35. davidbenari

    I Understanding 4-Momentum in General Relativity

    Conserved quantities in GR deal with ##p_\mu## not ##p^\mu## and while in Minkowski spacetime its easy to see what each of the components mean (since the metric is so simple) in general relativity I think its not and its starting to confuse me. Why exactly is ##-p_0## the energy in general...
  36. Seanra

    I My lecturer says "Special relativity is absolutely wrong"

    Hi guys In an assignment I wrote for university I was penalised for claiming that FTL neutrinos would violate special relativity. Below is the relevant part of my assignment and the response from my lecturer. Could somebody please explain what he could mean by that because as far as I can...
  37. P

    B Understanding Gravity & Orbits in Light of Relativity

    I've been re-reading Hawking's Brief book (for the fourth time :-) and trying to understand a bit more. He discusses how General Relativity tell us that the planets don't orbit due to gravity, but due to the bending of space-time by mass/energy. That in fact, they are actually following a...
  38. N

    I Understanding 4-Momentum in Special Relativity

    Hello, I am studing elementary particle physics and want to ask something, just to check if I have understood properly. So, as I understand, this is true about four-momentum in special relativity: 1. The square of the sum of particles' four momenta is invariant under Lorentz transformations...
  39. F

    I Proving Orthogonality of Vector w/ Schnutz Special Relativity Tensors

    There's a question in Schnutz - A first course in special relativity Consider a Velocity Four Vector U , and the tensor P whose components are given by Pμν = ημν + UμUν . (a) Show that P is a projection operator that projects an arbitrary vector V into one orthogonal to U . That is, show that...
  40. A

    I Stanford: objects in spacetime all move at constant speed c?

    In this http://web.stanford.edu/~oas/SI/SRGR/notes/SRGRLect6_2007.pdf, it is stated: Likewise, objects in spacetime all move at constant speed c in spacetime but if you change its direction, say by moving at speed v in the x direction, then spatial speed will change and so will the speed along...
  41. A

    I Deriving Einstein's Relativity Postulates from Minkowski Spacetime

    Does the constancy of the speed of light for all observers naturally emerge from the Minkowski spacetime metric? Do Einstein's two postulates of relativity emerge from the Minkowski spacetime metric? Suppose we begin with Minkowski spacetime and the Minkoswki metric...
  42. Anjum S Khan

    Einstein's theory of relativity

    A falling apple made Newton to think about gravity. What made Einstien to think and develop his theory of relativity ? What devices/apparatus/equipment did he use for his theory of relativity ?
  43. Perturbative

    I General Relativity within the confines of a Hilbert Space

    Introduction If Quantum Mechanics is more fundamental than General Relativity as most Physicists believe, and Quantum Mechanics is described using Hilbert Spaces wouldn't finding a compatible version of General Relativity that operates within the confines of a Hilbert Space be of utmost...
  44. LarryC

    I What specific momentum should be used in the matter wave?

    According to de Broglie's hypothesis, the matter wavelength could be described by lamda=h/p. But which momentum, the classical p=mv, or the relativistic one p=mv(1-v^2/c^2)^(-1/2) should be used in this equation? Additionally, an even more confusing issue to me is regarding the other relation...
  45. Holden Kenne

    B Understanding Newton's Laws of Motion: Exploring Errors and Contradictions

    Hi, I'm a frehshman in High school writing a paper about Isaac Newton. One of my paragraphs is about the laws of motion. I came across a tidbit of information - there are instances in which Newton's laws are not correct. One example I found was that, at speeds approaching that of light, an...
  46. ibkev

    General relativity math self-study - what next?

    My goal is to develop an intuitive understanding of the math underlying general relativity and ultimately be able to take a book like Wald or Carroll and, as someone on these forums commented once, “be able to casually read it while sipping my morning coffee and listening to the news.” :) So...
  47. Israel Eydelson

    How do we simulate time flow in video games? EE/PHYS

    Hi Everyone, I am very ignorant and uneducated but I have a few questions about a difficult thought experiment. 'How does time flow in a video game/simulation or even in our imagination in relativistic terms?' Let's say a processor operating at 4.2 Ghz, 42 billion instructions per second at...
  48. ChrisVer

    I Solving General Relativity's Schwarchzild & FRW Metrics

    I had the following question how are the Schwarchzild metric that describes a spherically symmetric matter distribution (such as a star) be compatible with the FRW metric that describes the 'overall universe' that the star resides in/is part of its matter distribution? Then we say that FRW...
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